1. Field of the Invention
The present invention relates to a fuel tank float valve located at an air bleed duct in an upper portion of the fuel tank. The valve bleeds air in the fuel tank during fueling, and also stops air flow if the fuel liquid level rises to the feeding-up position of the fuel tank.
2. Description of the Related Art
Heretofore, float valves that are located in fuel tanks are proposed in Japanese Patent Laid-Open No. 5-185850, Japanese Utility Model Publication No. 4-39061 and have a structure as shown in FIG. 1.
This type of float valve V0 comprises a float 5 positioned inside a cylindrical case 1 that is, in turn, fixed to an upper wall 9 of the fuel tank 8. In addition, the case 1 comprises a top wall 2 having a communication hole 2a that communicates with an air bleed duct 7, a bottom wall 3 that is located below the top wall 2, and a side wall 4 connecting the top wall 2 and bottom wall 4.
A through-hole 3a and an air hole 4a are formed in the bottom wall 3 and side wall 4 respectively in order to enable fuel F and air A to flow into the case 1.
A valve portion 5a, that can block the air bleed duct 7, is formed above the float 5, and a compression spring 6 is located between the float 5 and bottom wall 3.
Compression spring 6 supports float 5 so as to block the communication hole 2a in case a car inclines or turns over in an accident. That is, for example, if the car turns over, the communication hole 2a would lie below float 5. However, the spring load of the compression spring 6 is set so that the sum of mass of the float 5 and repulsive force of the compression spring 6 may be bigger than buoyancy of the float 5. Of course, the spring load of the compression spring 6 does not raise the float 5 to block the communication hole 2a before the fuel F rises in case the car is in a proper posture. However, the spring load of compression spring 6 is set within a range so the sum of the buoyancy of float 5 and the repulsive force of the compression spring 6 is bigger than the mass of float 5.
Therefore, in this type of float valve V0, if the fuel liquid level in fuel tank 8 rises in a normal manner, fuel F flows into case 1 through through-hole 3a and air flows through the air hole 4a, and the fuel raises the float 5. Therefore, since the valve portion 5a blocks the communication hole 2a when float 5 rises enough, the float valve can then prevent an outflow of the fuel F from the fuel tank 8. In addition, air that passed the communication hole 2a and air bleed duct 7 is sent to a canister (not shown).
However, rapid flow of fuel during filling can cause problems with use of a conventional type float valve V0.
Because air A rushes into the case 1 from the air hole 4a during rapid fueling, the flow velocity of the air A that is bled out to the air bleed duct 7 increases rapidly, and hence a flow of the air A generates lift and raises float 5. Also the fuel F flows into the case 1 from the through-hole 3a, in the bottom wall 3 of the case 1, so that float 5 rises by the lift caused by the fuel F. Then, before the fuel liquid level rises to a fuel level LF, where the float 5 is located at a proper valve-closing position where tank 8 is considered to be full, the valve portion 5a sometimes blocks the communication hole 2a due to the flow of the air A. This valve-closing operation easily occurs because the downward force acting on float 5 (a residual value after the lift of the float and the repulsive force of the compression spring is subtracted from the mass of the float) becomes small especially when the float generates lift caused by fuel F.
Furthermore, since such valve-closing operation suddenly stops the flow of the air A that is bled out of the fuel tank 8 during fueling, the valve-closing operation quickly raises internal pressure in the fuel tank 8. Thus, at the fuel inlet of the fuel tank 8, the valve-closing operation can result in additional problems.
That is, although problems do not arise where internal pressure in the fuel tank gradually rises, it is not desirable that the internal pressure in the fuel tank 8 the rises instantaneously or drastically as can occur if of the flow of the air A is stopped as above described. Because the internal pressure in the above state generates back pressure on the fuel extending to the fuel inlet, the back pressure suddenly activates an automatic stop mechanism in the fuel nozzle that detects and reacts to increased internal pressure in the fuel tank, whose level is more than the regular value, and acts. Consequently, the automatic stop mechanism stops fuel feeding prematurely, and hence the desired amount of the fuel to fill the tank will not be fed.
As a countermeasure to these issues, Japanese Utility Model Laid-Open No. 63-137033 and so forth propose to install a check-valve at the fuel inlet, but this is not desirable because the number of parts used in the fuel tank increases.